In many manufacturing and production processes, objects are subjected to a number of sequential chemical treatments to produce a desired product. For example, metal substrates may be treated with phosphates in order to improve their corrosion resistance and susceptibility to subsequent coating procedures. A typical phosphating process includes a number of baths into which a substrate is sequentially immersed. That substrate might intially be immersed in a cleaning or pickling solution, then in a phosphate solution and ultimately in a sealant solution. Between those immersions, the substrate may be immersed in rinsing solutions. In electroplating, chemical process control of numerous solutions may also be applied. There, a substrate may be initially immersed in a cleaning or etching solution. After the cleaning, metal layers are deposited electrolytically on the substrate in other solutions. Again, between immersions in different solutions, the substrate may be immersed in rinsing baths to avoid cross-contamination between the various solutions.
In these and other chemical processes employing multiple baths, it is important to control the characteristics of the baths to produce a product of acceptable and repeatable quality. For example, in phosphating baths and in electroplating baths, maintenance of the pH of the "active" baths within a limited range can be essential to good quality results. Other bath characteristics that may be of importance in some process steps include the concentration of various ions, both absolutely and in relation to each other, the electrical conductivity of the bath and the bath temperature. In rinsing baths, the concentration of ions built up over time from other baths is a significant characteristic. When contamination in a rinsing bath due to "active" baths exceeds acceptable levels, the rinsing solution may need to be changed or cleaned.
In "active" solutions used in electrodepositing, phosphating and cleaning, variations outside established ranges of pH, electrical conductivity and temperature may need corrective action. In the first two instances, the correction may be achieved by adding additional reagents to the solution. In the final case, an adjustment in the current flowing through an electrical heater or in the quantity of coolant flowing through a heat exchanger may correct a temperature variation.
In known chemical process control apparatus, samples can be periodically withdrawn from the baths and analyzed using automated chromatographic and electrochemical techniques. This analytical equipment may be controlled by a microprocessor that, in response to the results produced by the analytical instrument, can activate pumps and/or valves to add make-up chemicals to the baths.
Typically, microprocessor-based chemical process control equipment and the analytical instruments it employs are extremely complex. Therefore, a highly trained operator is required to monitor and control the equipment. In addition, known chemical process controllers are specific to particular analytical instruments. That is, removal of an analytical instrument, such as a chromatograph, and its replacement with a different analytical sensor is difficult. In known microprocessor-based systems, replacement of sensor type requires significant reprogramming.
Accordingly, it would be useful to provide an apparatus and method for chemical process control that can be operated by a relatively unsophisticated person. It would also be desirable to prohibit process control variable and system software alterations by the relatively unskilled operator. The desired system would permit more highly skilled, authorized persons, both off-site and on-site, to supply, as needed, various process control and software change functions. A desirable process control apparatus would permit changes in sensors without significant software changes.